1. Field of the Invention
The present invention relates generally to improvements to chemical vapor infiltration processes and apparatus for the preparation of reinforced ceramic matrix composites. More particularly, the present invention relates to the use of microwave energy heating during the production of reinforced ceramic composites in a non-steady state chemical vapor infiltration process.
2. Description of the Background Art
Chemical Vapor Infiltration ("CVI") is a well-known technique for the production of reinforced ceramic composites. The CVI process is capable of incorporating continuous ceramic fibers into a ceramic composite without mechanically, chemically or thermally damaging the fibers. In the CVI process, ceramic precursors (i.e., vaporized organometallic ceramic matrix precursor(s) and associated carrier and process gases, which are referred to collectively as "ceramic precursor reactant gasses" herein) infiltrate into a preform, such as a fibrous preform consisting of SiC, graphite and/or other ceramic fibers. With the application of sufficient heat, a chemical reaction occurs between the ceramic precursor compounds and other components of the reactant gas, whereby ceramic material is deposited upon the fibers, producing a two-phase matrix.
CVI processes typically are isothermal processes which require the reactant gases to diffuse into the fibrous preform. These processes are well-suited for the fabrication of thin-walled composites of complex shapes Usually, the process is controlled so that densification proceeds very slowly, in order to maximize the deep diffusion of the reactants into the preform and to permit the reaction by-products to diffuse out of the preform. A low deposition rate is intentionally maintained in order to prevent (to the extent possible) sealing the exterior surface of the preform before the interior is densified. Despite these efforts, significant density gradients typically are observed because of the greater deposition of material on the surface of the preform Even for thin composite structures of the order of one fourth of an inch (6-7 mm), very long processing times are required. For thick wall components, the processing time may exceed 500 hours, making this process extremely expensive.
A modified process for the fabrication of CVI ceramic-ceramic composites, which utilizes thermal as well as pressure gradients to reduce the infiltration time, is described in U.S. Pat. No. 4,580,524 (Lackey, Jr. et al.; Apr. 8, 1986). As described therein, a fibrous preform is placed within a graphite holder which is water-cooled. The reactant gases are introduced at the cool side of the preform. The other end of the preform is exposed to the furnace, creating a sharp thermal gradient across the preform. The gases introduced at the cool side, because of the pressure, continue to diffuse into the hot region, forming the matrix material on the fibers of the preform. The by-product is released and exhausted through the perforated lid into the hot zone. The deposition of the matrix material in the hot portion of the preform increases the density as well as the thermal conductivity of the fibrous preform. The increase in thermal conductivity allows the deposition region to move from the hot surface to the cooler surface. This process is limited by the permeability of the densified composite which prevents sufficient flow of the reactant gases into the preform because of the back pressure of the by-product released during the reaction. Nevertheless, this is an efficient method for fabricating composites having a small wall thicknesses. The production of more than one preform at the same time in the same furnace using this method is a serious engineering challenge, however, because of the thermal hydrodynamic complexity of such a processing configuration.
Another known chemical vapor infiltration process is characterized as a pressure variant, isothermal CVI process. The absolute pressure in the infiltration hot zone is rapidly varied or "pulsed" from nominally 5 to 25 torr. Flow resistance of the porous composite structure produces a lead/lag pressure differential, which although very small, significantly increases mass transfer over that obtainable by purely diffusion-related mass transport mechanisms. This process is described in U.S. Pat. No. 4,960,640 (Paquette, et al.; Oct. 2, 1990). The process provides only very slight depth of infiltration performance advantages over low temperature isothermal, isopressure chemical vapor infiltration, however.
For these reasons, fabrication of ceramic matrix composites using conventional chemical vapor infiltration processes is typically limited to composites where total wall thickness is on the order of 1/4" to no more than 3/8". Accordingly, there has been a need for further improvements to processes for producing reinforced ceramic composites, especially composites having wall thicknesses on the order of one inch or more.
In recent years there has been a growth in the use of microwave energy for the processing of materials. See Materials Research Society (MRS) Symposium Proceedings, Vol. 124 (Sutton et al., 1988) and Vol. 189 (Snyder et al., 1991); Ceramic Transactions, Vol. 21 (Clark et al., 1991). Important applications of microwave energy include ceramic drying, binder burnout, sintering of ceramics and composites, joining of ceramics, fabrication of optical fibers in a Modified Chemical Vapor Deposition (MCVD) station and combustion of reactants for the synthesis of ceramics and composites.
The combustion synthesis of ceramics and composites is an interesting application, providing evidence of reactions (ignition) initiating at the center of the sample, with the combustion wavefront propagating radially outward (Ahmad et al., J. Microwave power and Electromagnetic Energy, Vol. 26, No. 3, p. 128, 1991). The lower density compacts had higher heating rates and the ignition and wavefront propagation was much faster than in the high density compacts.
The potential advantages of microwave heating for use in isothermal and temperature gradient (i.e., with active cooling of the sample) CVI processes have been mathematically modelled and described via computer simulation under steady state processing conditions in Evans et al., A Mathematical Model for Microwave-Assisted Chemical Vapor Infiltration, Mat. Res. Soc. Symp. Proc. vol. 189, pp. 101-107 (1991). Evans et al. do not disclose or suggest the use of non-steady-state conditions, however, such as the use of cyclic or pulsed variation in the application of microwave energy and/or pressure variation within the reaction chamber.
Accordingly, the present invention provides improvements to CVI processes for the production of reinforced ceramic composites. These improvements will include improvements in the thickness of fabricated composites as well as improvements in the overall efficiency (for example speed and cost) of fabrication.